As is well known, polyblends of rubber with styrene/acrylonitrile polymers have significant advantages in providing compositions of desirable resistance to impact for many applications. Such polyblends are known as ABS polymeric polyblends having a matrix phase copolymer of monoalkenyl aromatic and alkenyl nitrile monomers having dispersed therein a conjugated diene rubber grafted with said monomers. Various processes have been suggested or utilized for the manufacture of such polyblends including emulsion, suspension and mass polymerization techniques and combinations thereof. Although graft blends of a monoalkenyl aromatic and alkenyl nitrile monomers and rubber prepared in mass exhibit desirable properties, this technique has a practical limitation upon the maximum degree of conversion of monomers to polymer which can be effected because of the high viscosities and accompanying power and equipment requirements, which are encountered when the reactions are carried beyond a fairly low degree of conversion after phase inversion takes place. As a result, techniques have been adopted wherein the initial polymerization is carried out in mass to a point of conversion beyond phase inversion at which the viscosity levels are still of practical magnitudes, after which the resulting prepolymerization syrup is suspended in water or other inert liquid and polymerization of the monomers carried to substantial completion.
N. E. Aubrey in U.S. Pat. No. 3,509,237 disclosed a mass/suspension method of polymerization styrene/acrylonitrile having diene rubbers dissolved therein with the rubber being grafted, inverted and dispersed as rubber particles under agitation. After phase inversion, the viscous mixture is suspended in water and polymerization is completed producing a polyblend in the form of beads.
Such mass/suspension processes are used commercially, however, present the economic problems of batch operations requiring long cycles at relatively low temperatures to control the heat of polymerization. Continuous mass polymerization processes have great economic advantages if they can be run at higher temperatures and higher rates with the necessary control of the great heats of polymerization. In the case of polyblends, the dispersed rubber phase must be formed and stabilized as to its morphology bringing it through the continuous polymerization of the rigid matrix polymer phase so that the physical properties of the polyblend meet exacting property specifications.
Various methods have been developed for the continuous mass polymerization of polyblends. Ruffing et.al., in U.S. Pat. No. 3,243,481 disclose a process wherein diene rubbers are dissolved in monovinylidene aromatic and alkenyl aromatic monomers and polymerized in four reaction zones. Such processes require physically separated reactors providing different reacting conditions for each step of polymerization involving costly multiple reactors and specialized equipment.
U.S. Pat. No. 3,337,650 discloses a continuous mass polymerization process for ABS polyblends wherein styrene and acrylonitrile monomers and a solvent having a diene rubber dissolved therein are continuously charged to a first back mixed stirred reactor operating at conversion of about 15 to 40% solids followed by further polymerization in a second back mixed reactor operating at 50 to 75% solids. Here, back-mixed stirred reactors running to high solids in the second reactor have the problem of handling the high viscosity experienced in large reactor processes. Because of the high viscosity, heat transfer also becomes a operational problem. Power and cooling problems require that such polymerization process, in large commercial operations, use solvents as diluents to reduce viscosity and aid temperature control. The use of solvents, in amounts of 15 to 20%, then creates the added problems of separating the ABS polyblend from the residual monomers and solvents and recycling such materials to the process on a controlled basis.
U.S. Pat. No. 3,511,895 discloses a process for the continuous polymerization of ABS polyblends wherein a styrene/acrylonitrile monomer solvent mixture is fed to a first back mixed reaction zone operating at least 20% conversion followed by a rubber/monomer solution providing a solids content of 25 to 50%, an effluent from the first reaction zone is fed to a second back mixed reaction zone operating at 55 to 70% solids and an effluent from the second reaction zone is fed to a third back mixed reaction zone operating at 70 to 90% solids with the ABS polyblend being separated from an effluent of the third reaction zone. This process is similar to that of U.S. Pat. No. 3,337,650 in that solvents or diluents are needed to reduce viscosity and control temperature.
There exists a need for a continuous mass polymerization process for ABS polyblends that can operate without solvents or diluents and with high conversion rates with a minimum number of reactors and low energy requirements. The towers and separate back-mixed reactors of the prior art used for the final polymerization in the range of 50 to 90% conversion have very low rates of polymerization, hence, require reactors that are substantially larger than the first reaction zone to provide the necessary hold-up time in the reactor to gain high conversion. Such systems have high energy requirements and relatively low polymerization efficiency.
It is the objective of the present invention to provide a process of high polymerization efficiency for ABS polyblends having superior properties.
It is also an objective of the present invention to provide a continuous purely mass polymerization process for ABS polyblends that can be operated to high conversion on a large scale without solvents or diluents for cooling and viscosity control.